The electrolysis of aqueous solutions of ionizable chemical compounds, particularly brine solutions, in a cell equipped with an anode and a cathode separated by a porous diaphragm is well known in this art. In most instances such cells are operated under conditions such that ionic migration and molecular migration through the porous diaphragm occurs to a substantial degree resulting in the contamination of the cathode liquor with undecomposed electrolyte and of the anode liquor with reaction products of the cathodic material and anodic materials.
It has been proposed to replace the porous diaphragm in such cells with a diaphragm impervious to both liquids and gasses thereby to control both ionic and molecular migration during electrolysis. Many patents, such as U.S. Pat. No. 2,967,807, U.S. Pat. No. 3,390,055, and French Pat. No. 1,510,265, disclose electrolytic cells incorporating as the diaphragm or barrier, membranes fabricated from synthetic organic ion-exchange resins. Among such resins, cation exchange resins of the "Amberlite" type, sulfonated co-polymers of styrene and divinyl benzene and others have been disclosed.
However, such resins have not been entirely satisfactory for one or more of the following reasons:
A. The resins are not stable to strong caustic and/or concentrated acidic solutions at temperatures above about 75.degree. centigrade.
B. The resins are effective only for relatively short periods.
C. The resins are expensive and fabrication costs are relatively high.
D. The voltage drop through the membrane becomes inordinately high as the caustic concentration in the cathode compartment increases to above about 200 gpl caustic.
E. Ion selectivity and chemical compatibility of the membrane decreases as the caustic concentration of the catholyte liquor increases.
f. Caustic efficiency of the electrolysis decreases as the caustic concentration in the cathode compartment increases.
In application Ser. No. 212,171 of Edward H. Cook, Jr. et al, filed Dec. 12, 1971, a process and apparatus is disclosed for carrying out the electrolysis of an ionizable chemical compound, specifically sodium chloride in a cell containing interposed between the anode and cathode thereof, a barrier composed of a single layer of a permselective membrane which is substantially impervious to liquids and gases, inert with respect to the electrolyte and products of the electrolysis and which is composed essentially of a hydrolyzed copolymer of tetrafluoroethylene and a sulfonated perfluorovinyl ether. This cell can be operated for extended periods without destruction of the diaphragm material and produces a caustic soda product which contains a low content of sodium chloride. However, it has been found that the caustic current efficiency has a tendency to decrease as the caustic concentration of the catholyte liquor increases above about 100 grams per liter ("gpl").
It can be seen that prior art procedures for electrolyzing aqueous solutions containing electrolysis wherein barriers or diaphragms comprising an ion-exchange substance are used to separate the cathode and anode compartments leave something to be desired.